1. Technical Field
The present invention relates to a switching mode power supply (SMPS) for controlling power of electronic machinery, and more particularly, to a low cost switching mode power supply that protects circuits from overvoltages and undervoltages by stopping a pulse width modulation operation.
2. Related Art
In general, an exemplary switching mode power supply (SMPS) utilizing a pulse width modulator controls power of electronic machinery by performing a line regulation on the primary side of power and a load regulation on the secondary side of power by means of a pulse width modulation controller. An example of such a switching mode power supply is disclosed in U.S. Pat. No. 5,448,469 for Switch Mode Power Supply with Output Feedback Isolation issued to Rilly et al. Generally, switching mode power supplies require a protection circuit to protect the load, or to protect components within the power supply from an overvoltage, an overcurrent, or an induced surge.
A switching mode power supply performs a line regulation by sensing the current from an output signal of a switching output part controlled by a pulse width modulation controller, performs a load regulation by detecting the voltage change induced from a coil of the largest turns ratio of the secondary loads of the power, and has the voltage of the power on the secondary side compensated in order to pass the results of the line regulation and the load regulation back to the pulse width modulation controller.
A switching mode power supply may include an alternating current (AC) input part, a rectifier part, a switching transformer, a switching output part, a pulse width modulation controller, a current sensing part, an output part, and an error detection and feedback part. The alternating current input part receives an alternating current voltage from an external source. Then the alternating current input part supplies the alternating current voltage to the rectifier part, which then outputs a direct current (DC) voltage to a load on the primary side of the switching transformer. The load in this instance is a primary coil. The switching transformer inputs the direct current voltage from the rectifier part. The switching transformer then induces different predetermined voltages to loads on the secondary side, each load having different turn ratios. The switching transformer induces the different predetermined voltages to the different loads by switching according to the control signal applied from a switching output circuit in the switching output part. The loads in this instance are coils.
The output part includes a plurality of outputs for the purpose of rectifying and smoothing voltages induced into the loads, and then outputs different predetermined voltages. Each output of the output part constitutes a rectifying diode and a smoothing capacitor for rectifying and smoothing voltages induced on the loads of the switching transformer. An error detection and feedback part detects an error of the highest voltage, induced on the load having the highest turns ratio, output from the output part, and feeds back the detected voltage errors to the pulse width modulation controller. A switching output circuit in the switching output part, which is composed of a switching element such as a field effect transistor (FET), controls on time and off time of the primary coil of the switching transformer by means of the pulse width modulation signal outputted from the pulse width modulation controller.
A current sensing part senses an output current from the switching output circuit and supplies the sensed current to the pulse width modulation controller. One example where a tertiary side coil is utilized is as follows. The pulse width modulation controller inputs at least three signals: a voltage error detected by the error detection and feedback part, the output current of the switching output circuit sensed by the current sensing part, and a voltage induced on a tertiary coil of the switching transformer. Then the pulse width modulation controller outputs pulse width modulation signals to the switching output circuit in the switching output part with a duty cycle modulated in accordance with the three signals received by the pulse width modulation controller.
The switching mode power supply senses an electric current from an output signal of a switching output part which is controlled by the pulse width modulation controller to perform the line regulation. The switching mode power supply detects a variation of voltage induced from a coil of one terminal among the loads at the secondary side of power to perform the load regulation. The results of the line regulations and load regulations are fed back to the pulse width modulation controller to thereby compensate the voltage at the secondary side of power.
Examples of power supplies utilizing pulse width modulation are disclosed in U.S. Pat. No. 5,633,787 for Switch-Mode Power Supply with Control of Output Supply Voltage and Overcurrent issued to Song, U.S. Pat. No. 5,594,631 for Digital Pulse Width Modulator for Power Supply Control issued to Katoozi et al., U.S. Pat. No. 4,876,637 for Power Converter and Method of controlling the Same issued to Mose et al., U.S. Pat. No. 4,916,569 for Short Circuit Protection for Switch Mode Power Supply issued to Konopka, U.S. Pat. No. 5,029,269 for Delayed Power Supply Overvoltage Shutdown Apparatus issued to Elliott et al., and U.S. Pat. No. 5,465,201 for Overload Protection of Switch Mode Converters issued to Cohen.
Examples of power supplies not utilizing pulse width modulation are disclosed in U.S. Pat. No. 5,124,906 for Multiple Transformer Switch Mode Power Supply issued to Kang, U.S. Pat. No. 5,216,585 for Switching Power Source Device issued to Yasumura, U.S. Pat. No. 5,657,218 for Switch Mode Power Supply Circuit issued to Rilly et al., U.S. Pat. No. 4,914,560 for Protection Circuit for Switching Mode Power Supply Circuit issued to Oh et al., U.S. Pat. No. 5,227,964 for Switching Power Supply with Overcurrent Protection Circuit issued to Furuhata, U.S. Pat. No. 5,570,277 for Switching Power Supply Apparatus issued to Ito et al., and U.S. Pat. No. 5,621,625 for Surge Protection Circuit for a Switching Mode Power Supply issued to Bang.
Currently there are some switching mode power supplies which claim to offer protection against overvoltages under certain conditions. However, I have discovered that it would be desirable to develop an enhanced low cost switching mode power supply utilizing a pulse width modulator in order to better protect against overvoltages and undervoltages.